Research On Key Technology Of Serial-resonant Combinatorial Waveform Emission System For Airborne Electromagnetic Method

Airborne electromagnetic method is a geological survey method which has advantages in both efficiency and speed.Compared with traditional methods,time-domain airborne electromagnetic methods more flexible,cheaper and more widely used.Our country has a vast territory and complicated national conditions.Traditional methods usually can't work well in all kinds of complicated landforms.While time-domain airborne electromagnetic method,equipped in an aircraft(such as a helicopter or a fixed wing aircraft)as a carrier,takes devices such as electromagnetic emission system,reception system,data processing system,pod,etc.It works via emitting the primary field and receiving the secondary field responded by the magnetism sensitive objects detected.The conductibility of the underground object is available after the data are analyzed.So this method is able to overcome the difficulties with no ground survey staff,and is especially suitable for complicated terrains like mountainous region,lake,forest,desert,swamp,etc.It's widely used in fields like surveys of advance of sea,detection of minerals and evaluation of environment.Time-domain airborne electromagnetic method has many kinds of waveforms,which mainly include trapezoid waveform,square waveform,half-sine waveform and triangle waveform.At present,all of the mainly representative time domain airborne electromagnetic emission systems have only one kind of current waveform emitted,which has very sole functions.Therefore,the emission circuit this paper researches is very multifunctional.It can emit both bipolar trapezoid waveform,which is suitable for exploring objects with a small time constant,and half-sine waveform,which is suitable for exploring objects with a large time constant.It can meet the requirements of both deep and shallow explorations,as well as both the on-time and off-time acquisitions due to very low switch noises.Based on the parameters of the airborne electromagnetic emission systems of international large-scaled,influential and advanced companies like Geotech and CGG,this paper compares the waveforms,frequencies and amplitudes of the systems and discusses the influences they caused.Then this paper introduces a double waveform emission system which consists of four parts:an emission coil,a separation circuit,a trapezoid wave transmitter branch circuit and a half-sine wave transmitter branch circuit,which share a common transmission coil.The two branch circuits are separated by the separation circuit that consists of switch devices like thyristors,making it possible for the two branch circuits to work without interferences between each other.The half-sine wave is produced through resonance.The current resonates in the RLC series circuits and has an approximate half-sine waveform.The trapezoid wave is produced through the control of IGBT,it consists of part of a half-sine wave and square wave,making it become a trapezoid waveform.Monitor circuits and signal processing circuits are introduced into the half-sine wave circuit and the trapezoid wave circuit in order to avoid circumstances like overvoltage between the capacitor and overcurrent in the circuit.Monitor circuits and signal processing circuits can shut down the system when an accident happens.A simulation system is built up via Matlab Simulink module to ensure the correctness of the design.And then a whole emission system is built in the laboratory.Tests on the main parts of the system and the system as a whole are done and further optimization of the circuit parameters are given.This paper firstly describes the design of the circuit that produces the two kinds of waveforms and the derivation of the mathematical model of the circuits,then offers the calculation method for certain parameters.The parameters of the system are tested when the system is complete.The test results have meet the requirements:the peak voltage of the half-sine wave is 1200A,the maximum output magnetic moment is 388800 Am~2,the width of pulse is 4.6ms;the peak voltage of the trapezoid wave is 150A,the maximum output magnetic moment is 48600 Am~2,the width of pulse is 2ms.The frequency of the fundamental wave of the double wave is 25Hz,and the frequency of the fundamental wave,as well as the time interval between the half-sine wave and the trapezoid wave are all controllable.Finally,this paper summarizes the experience and difficulties encountered during the design of the system,and gives some suggestions for further research to overcome the shortcomings of the emission system.